1. Field of the Invention
The present invention relates generally to scratch protection for integrated sensor circuits, and more specifically to improvement of scratch protection in capacitive sensor circuits through planarization of passivation overlying capacitive sensor electrodes.
2. Description of the Prior Art
Fingerprint acquisition circuits employ arrays of sensors coated with a dielectric onto which the finger is placed with the epidermis in contact with the dielectric. The ridges and grooves on the epidermal layer of the finger are then detected by the sensors, which transmit signals representative of the detected pattern. Although various sensors are possible (e.g., resistive, etc.), capacitive sensors have been found to provide the best performance and security. Since capacitance between two capacitive plates is inversely proportional to the distance between the plates, using the contacting dermal tissue itself as one capacitor plate and the sensor electrode as the other and then determining capacitance for each sensor electrode in the array, it is possible to locate the ridges and grooves of the fingerprint.
Such capacitive sensors cannot be mechanically protected because physical contact on the surface of the integrated circuit with the finger is necessary. However, some scratch resistance protection for the capacitive sensor electrodes is required to prevent xe2x80x9cscratchxe2x80x9d damage to the sensor electrodes. Such damage typically results from undue (and unnecessary) pressure on the surface of the integrated circuit, alone or in combination with some sharp edge or protrusion such as a callous or scar, fingernail, rings, dust or dirt particle, etc. While extremely hard passivation stacks employing silicon carbide (SiC) have been developed for these circuits, the capacitive electrodes may still become very badly damaged through use.
It would be desirable, therefore, to improve the scratch resistance of passivation employed in capacitive sensor circuits.
Passivation for capacitive sensor circuits, which overlies the capacitive sensor electrodes and is normally conformal to the electrodes and the underlying interlevel dielectric, is planarized by forming a layer of flowable oxide over the electrodes before forming the passivation. The flowable oxide, which is preferably very thin over the electrodes to minimize loss of sensitivity, provides a substantially planar upper surface, so that passivation formed on the flowable oxide is also substantially planar. Alternatively, a deposited oxide planarized by chemical mechanical polishing may be employed to planarize the surface on which a passivation stack is formed. The planarized passivation provides markedly improved scratch resistance.